A subshell is a set of electrons that all have the same value of (azimuthal quantum number). You can think of as a label for a group of orbitals that have related shapes.
A subshell with =0 is called an s subshell and contains spherical orbitals; a subshell with =1 is called a p subshell and contains dumbbell shaped orbitals; a subshell with =2 is called a d subshell and contains (mainly) orbitals that look like 4-leafed clovers.

The number of possible orbitals in a subshell is determined by the number of possible m values for the subshell. Think of m as a label for individual orbitals within the subshell. For every m value that is allowed, there is one orbital in the subshell.

Allowed values of m are integers between - to +, including zero.

The number of possible m values determines the number of orbitals in a subshell.

possible values of m

number of orbitals in this subshell

0

0

1

1

-1, 0, +1

3

2

-2, -1, 0, +1, +2

5

3

-3, -2, -1, 0, +1, +2, +3

7

Each orbital can hold a maximum of two electrons, so the maximum number of electrons you can put into the subshells are
2, 6, 10, and 14 for s, p, d, and f subshells, respectively.